Diverging shell antennas often employ waveguides to supply input signals. In such configurations, a dominant mode, such as a TE.sub.11 mode in a circular waveguide, is used as the input signal. Such modes are generated in the waveguide from an external source in a manner known in the art.
In the absence of any other elements the TE.sub.11 mode propagates from the waveguide through the diverging shell to the distal end of the diverging shell. The signal then exits through the antenna aperture and travels to the far field. Desired antenna performance characteristics such as gain, sidelobe levels, bandwidth, and E-plane and H-plane field strength distributions are often not achievable using this configuration. It is known that the performance or characteristics of an antenna can be adjusted by controlling a combination of modes at the distal end of the diverging shell. For example a high gain relatively narrow beam antenna pattern can be achieved by combining HE.sub.11 with TE.sub.12 and TM.sub.12 modes.
It is therefore desirable to convert the dominant TE.sub.11 mode supplied to the waveguide to a controlled combination of HE.sub.11 and higher modes at the output aperature.
There are a number of methods of converting the dominant TE.sub.11 mode supplied in the waveguide to a controlled set of modes in an output aperture. Where the dominant mode is a TE.sub.11 mode in a circular waveguide, conversion of the TE.sub.11 mode into an HE.sub.11 mode within the waveguide is often employed as a first step.
This conversion can be achieved by a number of techniques such as using one of many forms of "reactive" surface for the outer wall of the circular waveguide. Typical "reactive" surfaces used for this purpose are metal corrugations, dielectric coated wire adjacent to an outer conducting surface, or a thin dielectric sleeve with an outer conducting surface. Another technique is the use of a dielectric rod positioned to be axially symmetrical with the waveguide. Where the cross-sectional geometry is chosen appropriately and a sufficient length is chosen, a conversion of the dominant TE.sub.11 mode to the dominant HE.sub.11 mode will occur, as is known in the art. In this manner, the dominant HE.sub.11 hybrid mode is produced within the circular waveguide and feeds the diverging shell.
Where waveguide-fed diverging shells use an HE.sub.11 mode as the input to the diverging shell, various techniques are employed to achieve a combination of known higher-order modes at the output aperture. For example, one prior art device utilizes a diverging shell having a multi-sectional construction. The shell diverges at an initial half-flare angle for a distance and then the half-flare angle approaches 0 degrees, forming a discontinuity in the wall of the diverging shell. Divergence resumes at a point further along the wall forming a second decontinuity. The flare angles and separation between discontinuities, or flare angle changes, are chosen to establish the desired relative phase and amplitude of the various modes such as to produce the desired radiation pattern characteristics. Because the shell wall discontinuities are fixedly incorporated in the diverging shell, tuning of the antenna by relocating the discontinuities is not achievable without completely restructuring the diverging shell.
In the prior art, the generation and relative phase relationships of the higher-order modes are determined by fixed elements or by elements not readily changeable. No adjustment of the relative modes for a given antenna configuration is contemplated. Further, none of the above utilizes a simply positioned, slideable element that can be slideably altered and adjusted to generate and control the phases of the various modes to achieve the desired antenna performance characteristics. As a result the performance or characteristics of an antenna cannot be adjusted after manufacture to optimize the antenna for the particular use nor can an antenna design be simply changed at low cost and experimentally verified for some new purpose prior to manufacture.